The present invention relates to methods of making silicone foams having reduced density and foam compositions made thereby.
Silicone foams are known in the art and have been produced over a broad spectrum of densities, for example, as described in U.S. Pat. No. 3,070,555, U.S. Pat. No. 3,338,847, U.S. Pat. No. 3,425,967, U.S. Pat. No. 3,677,981, and U.S. Pat. No. 3,730,931.
A useful new class of foamable silicone compositions comprising a hydroxylated organosiloxane, an organohydrogensiloxane and a platinum catalyst is described by Smith, U.S. Pat. No. 3,923,705. Smith discloses that organosiloxane foams can be prepared by mixing an organohydrogensiloxane having an average of at least three silicon-bonded hydrogen atoms per molecule, an average of no more than one silicon-bonded hydrogen atom per silicon atom and organic radicals selected from the group consisting of alkyl radicals having from 1 to 6 carbon atoms per radical, phenyl and 3,3,3-trifluoropropyl radicals; a hydroxylated organosiloxane having an average of from greater than 1.0 to 2.5 silicon-bonded hydroxyl radicals per molecule and having an average of at least one organic radical per silicon atom selected from the group consisting of alkyl radicals having from 1 to 6 carbon atoms per radical, phenyl and 3,3,3-trifluoropropyl radicals; and a platinum catalyst in an amount of from 5 to 200 parts by weight platinum per one million parts by weight total composition, said organohydrogensiloxane and said hydroxylated organosiloxane being present in sufficient amounts to provide a molar ratio of silicon-bonded hydrogen atoms to silicon-bonded hydroxyl radicals of 2.5 to 40, where a mixture is obtained having a viscosity of less than 100,000 cs. at 25.degree. C.
Lee and Ronk found that the foams of Smith were essentially closed-cell in structure, and in U.S. Pat. No. 4,026,842 disclosed that the open-cell structure could be increased by having present in the foamable composition an organomonohydrogensiloxane. Such organomonohydrogensiloxane has one silicon-bonded hydrogen atom per molecule and organic radicals selected from the group consisting of alkyl radicals having from 1 to 6 carbon atoms per radical, phenyl and 3,3,3-trifluoropropyl, said organomonohydrogensiloxane being present in an amount sufficient to provide a molar ratio of silicon-bonded hydrogen atoms from the organohydrogensiloxane to silicon-bonded hydrogen atoms from the organomonohydrogensiloxane of from 0.4 to 2:1 and the molar ratio of total silicon-bonded hydrogen atoms present in the mixture to silicon-bonded hydroxyl radicals remaining within the limits of 2.5 to 7:1.
It was disclosed by Modic in U.S. Pat. No. 4,189,545 that one disadvantage of compositions formed according to Smith is that good foams are not always produced unless an extremely active platinum complex catalyst is utilized. Modic overcame such shortcoming by including from 100 parts per million to 1.5 parts by weight of water as a blowing agent in a composition comprising 100 parts by weight of a vinyl-containing diorganopolysiloxane wherein the organo groups are selected from the class consisting of alkyl radicals of 1 to 8 carbon atoms, aryl radicals, vinyl radicals and fluoroalkyl radicals of 3 to 8 carbon atoms and such that the polymer contains from 0.0002 to 3 percent by weight vinyl radicals and the viscosity varies from 100 to 1,000,000 centipoise at 25.degree. C.; 0 to 200 parts by weight of a filler; 1 to 50 parts by weight of a silicon hydride polymer wherein the organic radicals are selected from alkyl radicals of 1 to 8 carbon atoms, aryl radicals and fluoropropyl radicals of 3 to 8 carbon atoms, the hydrogen content varying from 0.3 to 1.6 percent by weight and the viscosity ranging from 5 to 100 centipoise at 25.degree. C.; and from 1 to 250 parts per million of a platinum catalyst.
Those skilled in the art recognize that the viscosity of a foamablle silicone composition impacts upon the density of the cured composition. That is, the higher the viscosity of the uncured composition, the lower the density of the resulting foam, or conversely, the lower the viscosity of the uncured composition, the higher the density of the resulting foam. Inasmuch as compositions having a lower viscosity are more easily mixed and dispensed than compositions of higher viscosity, it is desirable to provide methods for reducing the density of foamed low viscosity compositions.
One such method is provided by Kim, Lee and Ronk in U.S. Pat. No. 4,026,845, which describes a method for reducing the density of silicone foams of the type described by Smith in U.S. Pat. No. 3,923,705. Therein it is disclosed that mixing a fluorinated surfactant with a foamable silicone composition results in a cured foam having lower density than if the fluorinated surfactant had not been included. Increasing the amount of fluorinated surfactant further decreases the foam density. Thus, for example, by increasing the amount of fluorinated surfactant from 0.01 up to 10 parts by weight in a composition based on 100 parts by weight of polydiorganosiloxane, the result is that the cured foam has a lower density at 10 parts by weight than at 0.01 parts by weight of fluorinated surfactant.
Modic, in U.S. Pat. No. 3,424,967, discloses a silicone foam comprising, by weight, 100 parts of a vinyl chainstopped diorganopolysiloxane of the formula ##STR1## where R and R.sup.1 are monovalent hydrocarbon radicals free of aliphatic unsaturation, with at least 50 mole percent of the R.sup.1 groups being methyl, and n has a value of from about 50 to 2,000 inclusive; from 10 to 100 parts of an organopolysiloxane copolymer comprising (R.sup.2).sub.3 SiO.sub.0.5 units and SiO.sub.2 units and in which from about 2.5 to 10 mole percent of the silicon atoms contain silicon-bonded vinyl groups, where R.sup.2 is selected from the class consisting of vinyl radicals and monovalent hydrocarbon radicals free of aliphatic unsaturation; from 10 to 100 parts of an inorganic fibrous material selected from the class consisting of asbestos and fibrous potassium titanate; from 0 to 50 parts of an inorganic filler; a platinum catalyst; an amount of a liquid organohydrogenpolysiloxane and from about 1 to 5 parts of a blowing agent. The skilled artisan will appreciate that the resinous copolymer of (R.sup.2).sub.3 SiO.sub.0.5 units and SiO.sub.2 units is included as a reinforcing material or to impart additional strength to the final foam and not for regulating foam density.
In U.S. Pat. No. 3,730,931 Simoneau et al. disclose a composition which can be mechanically frothed with air to produce a foam comprising vinyl chloride polymer, a plasticizer and a silicone resin uniformly mixed with the polymer and plasticizer, such resin being selected from the class consisting of silicone resins having R.sub.3 SiO.sub.0.5 monofunctional units and SiO.sub.2 tetrafunctional units with the ratio of monofunctional units to tetrafunctional units ranging from 0.25 to 0:75:1 and silicone resins having R.sub.3.sup.1 SiO.sub.0.5 monofunctional units, SiO.sub.2 tetrafunctional units and R.sub.2.sup.2 SiO difunctional units with the ratio of monofunctional units to tetrafunctional units being from 0.25 to 0.75:1 and the ratio of difunctional units to tetrafunctional units being from 0.05 to 0.1:1 where R, R.sup.1 and R.sup.2 are lower hydrocarbon monofunctional radicals. The composition preferably contains 20 to 40% vinyl chloride, 20 to 50% plasticizer and 1 to 8% silicone resin. To produce more efficient foaming the resins are further reacted such that they have a hydroxy content of less than about 0.5% by weight. Thus, the artisan will appreciate that an MQ resin is employed as a frothing or foaming agent in such a mechanical system because it is incompatible or insoluble in the base composition and not to reduce the density of the resulting foam.
Thus, it was highly surprising that resinous copolymers of R.sub.3 SiO.sub.0.5 units and SiO.sub.2 units or resinous copolymers of R.sub.3 SiO.sub.0.5 units, R.sub.2.sup.1 SiO units and SiO.sub.2 units could be utilized in the compositions of both Smith, U.S. Pat. No. 3,923,705, and Modic, U.S. Pat. No. 4,189,545 to reduce the density of the resulting foam.
Accordingly, it is an object of the present invention to provide a method of reducing the density of silicon foams.
It is also an object of the present invention to provide compositions which cure to foams and which have a resinous copolymer contained therein so that the cured foam has a lower density than those which do not contain such resinous copolymer.
Other objects and advantages of the present invention will be obvious from the following detailed description.